1. Field of the Invention
The present invention relates to a liquid crystal display apparatus for displaying images or pictorial images.
2. Description of the Related Art
To improve the resolution or display precision of a liquid crystal display apparatus for displaying images or the like, one frame needs to be divided into as many picture elements as possible. In other words, the liquid crystal display apparatus needs to have a display panel comprising a very large number of pixels. To efficiently drive many pixels, the active matrix method is widely employed, in which switching devices provided in the individual pixels are matrix-driven so as to turn the pixel electrodes on and off.
According to the types of switching devices, the active matrix method fall into two main groups: the three-terminal method, and the two-terminal method. Liquid crystal display apparatuses employing three-terminal devices, particularly, liquid crystal display apparatuses employing thin film transistors (referred to as "TFT"s hereinafter) formed by using amorphous silicon or polycrystal silicon, have been studied and developed.
FIG. 11 is a schematic sectional view of a display portion of a conventional liquid crystal display apparatus employing TFTs. The figure shows: a transparent substrate 1 made of, e.g., glass; an active layer 2 to 4 of a TFT (regions 2 to 4 are referred to as a "source", a "drain" and a "channel", respectively); a gate 5 of the TFT; a source line 6 made of ITO (indium tin oxide) or a metal such as Al; a pixel electrode 7 generally made of ITO, the pixel electrode 7 being connected to the drain 3; insulating layers 9, 9' and 9"; alignment control films 10 and 10' for aligning liquid crystal molecules; a liquid crystal 11; a counter electrode 12; and a counter substrate 13. In a normal production method, the two substrates 1 and 13 are formed separately, and the liquid crystal 11 is enclosed in a gap between the substrates while the gap size is being controlled by a spacer (not shown). The molecules of the liquid crystal 11 sandwiched between the substrates 1 and 13 are orientated in accordance with electric field which is controlled by switching of the TFT, thus changing the light transmission characteristic thereof.
However, because the gap size d.sub.1 in a pixel portion is substantially greater than the gap d.sub.2 size over the TFT as shown in FIG. 11, the electric field E.sub.1 in the pixel portion is different from the electric fields E.sub.2 and E.sub.3 over the TFT (E.sub.1 &lt;E.sub.2, E.sub.3). Therefore, the electric field E1 in the pixel portion is strongly affected by the neighboring electric fields E2 and E3, thus causing deterioration of the image quality or other adverse effects. Further, the difference between the gap sizes d1 and d2 directly affects orientation of the liquid crystal molecules and, therefore, also causes deterioration of the quality of a displayed image.
Japanese Patent Application Laid-Open No. 62-247330 discloses a method of substantially eliminating the above-described gap size difference by providing an insulating layer between the two pixel electrodes. However, this method cannot be simply applied to a display apparatus employing TFTs. Production of semiconductor active devices, such as TFTs, requires complicated and elaborate processes and, therefore, makes the quality control thereof very difficult.
Further, in a TFT formed as described above, two contact portions where the source region 2 and the drain region 3 are connected to a connecting line of the source line (signal line) 6 and the pixel electrode 7 respectively are surrounded by the insulating layers 9 and 9' and, thus, have the shapes of deep wells. Therefore, it is difficult to achieve good connection in the contact portions. Particularly, good connection between the pixel electrode 7 made of ITO and the drain region 3 is likely to fail, resulting in an increased contact resistance. Because ITO, which is used to achieve the essential condition of pixel electrodes, that is, transparency, has a low film forming temperature of about 150.degree. to 300.degree. C., filling the well-like contact portion with ITO (referred to as "step coverage" hereinafter) often fails and connection to the drain portion 3. Further, because the gap size between the substrates greatly vary over at the contact portions, disturbance of the liquid-crystal orientation characteristic and the display characteristic may well deteriorate. If ITO is also used to form the source lines 6, the same types of problem will occur, thus reducing the yield.